This invention relates generally to switching of data in communications and more particularly to systems and methods for switching multi-rate communications.
In telecommunications, there are a number of standard frame formats which are built from successive combinations of fundamental 64 kilobits/second (KBit/sec.) channels, referred to as communications. One such format is SONET (Synchronous Optical Network) which creates 125 us frames containing virtual tributary (VT) positions, referred to as VTs. A SONET frame includes a limited number of timeslots. The size of the VTs depends on an underlying communication being transported within a SONET frame. Examples of VT sizes include VT1.5, VT2, VT3 and VT6. A VT1.5 can be divided into three equally sized portions, each of which can be called a VT0.5.
A modified version of a SONET STS-1 frame is a system data format (SDF) frame. Within the SDF frame, there are modified versions of VTs. The modified versions are referred to as T virtual tributaries (TVTs). Within the SONET STS-1 frame, VTs are used, and within an SDF frame, TVTs are used. A TVT is equivalent to a VT. Examples of TVT sizes include TVT0.5, TVT1.5, TVT2, TVT3 and TVT6. A DS1 communication occupies 3 TVT0.5s. An E1 communication occupies 4 TVT0.5s and occupies a set of timeslots, such as, for example, columns, n, n+24, n+48, and n+72, within an SDF frame.
A switch fabric that includes cascading time and space switch elements manipulate VTs contained within a SONET frame. For manipulating the VTs, any VT in a SONET frame that is input to the switch fabric should be mapped to any same-sized VT in a SONET frame output from the switch fabric. Examples of configurations of the switch fabric include a T-S-T configuration and a T-S-S-S-T configuration, where T represents a timeslot interchange and S represents a space switch.
Some switch fabrics typically do not support different types of communications in an efficient manner. For example, a switch fabric that has a T-S-T configuration optimized to support a single communication type, may not support mixed DS1 and E1 communications without blocking the E1 and DS1 communications during transit. In the example, although the DS1 and E1 communications may transit through the left timeslot interchange T within the T-S-T configuration, a set of timeslots within the space switch S may not be empty to channel the DS1 and E1 communications. In such a case, the DS1 and E1 communications are blocked until the set of timeslots within the space switch S becomes empty.
Moreover, some switch fabrics cannot handle different types of communications without replacing hardware, such as timeslot interchanges and space switches, within the switch fabrics. For example, if a switch fabric includes timeslot interchanges that support VT1.5s, the switch fabric cannot support an E1 communication without replacing the timeslot interchanges with timeslot interchanges that support VT2s.